Tuesday, September 04, 2007

Lighting 102 - 3.1 Balancing Light: Twilight

For the available-light photographer, the idea of exposure is a fairly simple and static concept.

There is a correct exposure for a given ambient light scene. Sure, you can tweak it, say, half a stop up or down. But go much beyond that, and you move beyond "artistic license" to "I screwed up."

But what is the correct exposure when your photo can have as many different zones of varying light levels as you have flashes? The correct exposure is what you say it is. And you say it by establishing a zone of (traditionally) correct exposure on your main subject using the flash.

If you are looking for a touchstone in this process, that's it. Establishing a correct exposure on your primary subject allows you to do whatever you want with the exposure levels in the rest of the frame. And you can go far beyond the "correct exposure" range of an ambient-only, evenly lit scene. And look like you knew what you were doing.

In short, "screwed up" becomes "artistic license" when you have established an exposure reference point with your light on your main subject. (More after the jump.)

To really understand the concept of balancing light, many of you will have to expand your concept of a so-called proper exposure. After all, you are creating a scene that has precisely the tonal range that you want it to. You can use this ability to compress the tonal range of a photo, or to expand it. It's up to you.

Take this scene photo which includes long-time Strobist reader Ryan Brenizer. Exposing for the model, the sky is washed out. Exposing for the sky, the model would be too dark.

But with flash, you can expose correctly for both. By adjusting the shutter speed and aperture to get exactly the desired tone in the sky and then filling the model with flash sufficient to raise her exposure to the aperture you happen to be using, you get this:

In addition to turning the water into diamonds with his flash, Ryan has compressed the tonal range of this scene to where everything fits in the histogram rather nicely, thank you.

So, is Ryan shooting at the correct exposure? Yes. Or no, depending on exactly how he wants the background to look.

Ryan shot this photo at 1/250th at f/3.2 at ASA 160. He could easily open his shutter speed up to, say, 1/125 and lighten the background. Or, he could up the power on his flash by a stop, close his aperture down a stop (to f/5.0 - a partial stop between f/4.0 and f/5.6) and reset his shutter to 250th to darken the background.

How does that work? Let's look more closely.

The background is lit by ambient light. It is controlled by a combination of the aperture and the shutter speed. The model is exposed by the flash. (She would be significantly underexposed without the flash.) So as long as the model is receiving the correct amount of light from the flash, the background can be placed at whatever tone the photographer wants.

What if Ryan cranked up the power on his flash 2 and 1/3 stops to where it lit the model to f/8? (He would then set his aperture to f/8 to correctly expose her.)

But what about the shutter speed? The new shutter speed to get the same effect on the background would be 1/50th of a second. (We simply open up the shutter 2 1/3 stops to neutralize the fact that we closed down the aperture 2 1/3 stops.) Thus, the exposure on the background has not changed.

We did this step to get away from our 1/250th of a second sync speed, and give us some "playing around" room with the shutter speed.

So now, imagine you are Ryan, wading in the water, shooting at 1/50th at f/8 and getting the same tones as we see above. Now, say you drop the shutter to 1/100th. What happens?

Model lady does not change. She wants f/8 from the flash and that is what she is getting. But the background gets one stop darker. You have just increased the contrast range of the photo. Darker, moodier and looking completely different. And I'm thinking those water diamonds are really popping now.

Drop the shutter down to 1/200th. Darker still -- but not black yet. Completely different feel to this photo than with the other two.

Which is correct? They all are -- just different. "Correct" is determined by the exposure on the model -- and that is set by the flash (and choosing the corresponding aperture that makes her look well-exposed.)

But the sky? That's up to you. Airy, normal, moody, black -- it's all good. And it is all available to you.

What you have is two different photos -- each with its own exposure -- being compressed into one scene. There is a flash exposure, which happens instantaneously and is controlled with the aperture. Then you have an ambient exposure which happens over time and is controlled by a combination of the aperture and the shutter speed._____________________

Now, You Do it

Our first light balancing exercise will be very similar to Ryan's setup, except that you will probably stay dry and you probably won't have a beautiful model to work with. (If you want to stick a gorgeous model into the water to do this, knock yourself out.)

Drag a partner out to an area where you have a fairly low horizon and a view of the western sky. Go out at about sunset and wait for the twilight sky to meter (continuous light level) at your sync speed (probably a 250th) at f/5.6 at a reasonable ASA (ASA 200 or 400.)

Now shoot a photo of your model using the correctly exposed twilight sky as a backdrop. He/she will be too dark.

Next, light your subject with a flash so that he/she is correctly exposed at f/5.6. You can do this with hard or soft light, on-camera or off-camera light -- I don't care. We are working on balancing light here.

Shoot a few frames of your subject this way. Talk with them. Tell them how good they look. Show them the images on the back of the camera. You are doing this to (a) build rapport and (b) to keep them around for a few more minutes.

But now, also shoot some frames at 1/125, to underexpose the background by a stop. And try a few at 1/250th to underexpose by two stops. You should see a very different feel in these photos, but they should all look okay, as does the underexposed sky in this photo, by Jonathan Shears.

When the background drops to 1/30th instead of opening up the shutter to compensate, turn the power on your flash down by one stop. (If you were shooting at 1/4 power, move to 1/8.) Now, instead of opening up from 1/60th at f/5/6 to 1/30th at f/5.6, you're opening to 1/60 at f/4 and adjusting your flash to compensate. This buys you more shooting time before you get into the "Hail Mary" range of shutter speeds.

The next time the sky drops another stop in exposure, power down your flash another stop and move to f/2.8. if this sounds difficult, it is not. Try it.

As your light drops lower still, keep opening up your shutter. Play with different speeds to see the effect on the background. But remember to choose the correct aperture to expose your subject correctly with the flash.

You will soon have too little light to focus. But before that happens, you'll have a lot of cool photos, with a range of background looks.

If you want to post some, tag them:

StrobistLighting102BalanceTwilight

You can see the tagged photos here. If you would like to talk about it, I have set up a thread here.

There's lots more coming on the light balancing front, so no need to get fancy yet. Next week, we'll be looking at how to do this kind of thing in full daylight.________________________

35 Comments:

the lightning stuff that you have written my be correct from your point of view but not correct always.You need something inside you to make things happen.Its just not like that makes your iner feeling always right.I finally got tired reading your few may be wrong post.So i wrote this.Sorry for this.If you think that i was wrong just come around and tell me.

Not quite sure I understand everything you are trying to say. But no worries -- you shouldn't ever feel reluctant about expressing an on-topic (i.e., non-spamming) opinion in a civil manner on this site. Good luck find what you are looking for.

And you are right -- light (which is what I think you meant to say) is not everything. It's just what we write about around here.

This is the assignment I've been waiting for. I really hope I get my dslr body back from Nikon so I can play along at home otherwise I may have to do this with film! I'm glad I just bought that leaf shutter lens for my 645, syncs at 1/500th sec!

BTW, David this is by far one of the best explanations on balancing ambient with flash I have read and I've read quite a few.

This is the one thing I still haven't been able to figure out: once you've got the model dialed in at f/8 and a shutter speed of 1/50, why doesn't she change when you go to 1/100?

I know that the post says that she was being exposed for f/8, but wasn't she being exposed for the combination of f-stop and shutter speed? Why wouldn't she get darker when you start dropping the shutter speed?

Unless you're upping your flash power as you drop the shutter speed . . . then it makes perfect sense to me.

Or maybe it's that the light source you're using for her (the flash) is bright enough that the drop in shutter speed really changes the background, but only very slightly changes her?

By the way, David, thanks so much for the site. I've learned a ton, and I love my flashes now.

This was a sticking point for me as well before the lightbulb went on. The flash fires in about 1/1000 of a second. If your shutter is 1/50 or 1/500 you still capture the complete flash emission. The only thing that makes it brighter (or less bright) is changing the aperture or the flash power.

When I realized I could expose foreground and background independently life got much more complicated, but holy crap do those pics look good! As always, thanks David!

Now you speaka my language:) This is what originally got me into strobism, and I'll admit, it's a big part of what keeps me going. Being able to balance the human/nature aspect of an image is magical to me. To be able to include two of the things I like to shoot most in one shot, sunsets and people, is the best gift you could have ever given me.

Before, I could never really put into words the process I went through when doing this kind of shooting. It just happened. It wasn't until I attended your seminar that certain things hit me. Thanks again.

The reason the subject doesn't change when you go to 1/100 is because the duration of the flash is faster than the shutter speed. You could run up to 1/500 -- or your max sync speed -- and the subject won't change at all because:- the strobe duration is probably about 1/10000 sec (somewhere within that 1/60 or 1/100 sec); and- the strobe intensity hasn't changed; and- your aperture isn't changing

This also means that you can underexpose or overexpose your subject by tweaking the aperture or the flash power or both.

I think this was posted here before, but for a quick refresher, have a look at this video:http://www.studiolighting.net/digital-photography-one-on-one-e001-key-shifting-and-shutter-speed/

You mention toward the end, "next week we will talk about..." which implies one week to do this, but I was kinda hoping for a two week window to turn this around?? If not I'll try andre-work my schedule as this looks awesome.

AH! That makes perfect sense to me, and it's so obvious that I should have figured it out myself.

I appreciate both of you taking the time to make it clear to me.

By the way, what a gorgeous amount of possibilities this opens up. Get the subject right, and as long as you aren't running up to your maximum sync speed, you've got room to wiggle with your ambient exposure.

Of course, I can see problems (or artistic effects) if you go too slow with the shutter.

The model is lit by ambient light as we ll as the flash, of course, so in reality when you change the shutter speed, the exposure of the model does change, and a proportion of the flash and ambient. Right?

So if our shutter speed for the background is causing the model to be underexposed by one stop, we fill that one stop with the flash. And as stops are exponential (one step doubles or halves the amount of light), the model is half lit by the flash, and half by the ambient light. And so, if we drop our shutter speed to darken the background by one stop, we will also darken the model by one half of one stop. Which, since our eyes are pretty forgiving/insensitive to smaller light variations, is not enough for us to actually worry about. Am I right on this?

As beginning strobist I feel I'm nailing the exposure and flash possibility's indoors quit well.. but outdoors was still a bit difficult. This explanes a lot for me.. This weekend I will go out and have a try ;-)

Before reading that article I thought "Yes, now he'll clarify my question" but unfortunately not ;)I'm still not ready signing up at the Flickr groups so I'll try to adress that here.Please, maybe someday in your 102 tell us more about the advanced gel possibilities ;)The color correction thing with artifical light is pretty clear, also to use lets say a 1/4 CTO at daylight but whenever I tested other gels from the rosco swatchbook I always got unnatural looking light. So if you don't own a light meter and absence of enough experience it's quite hard to find the right gel.In similar situation like in that article I shot with the wrong gel. When color correcting you can choose between unnatural skintones or background.I sure I'm not the only one who would like to learn more in that direction so this is my suggestion for a future subject.

I haven't yet tried these techniques myself, but just from a viewer's eye (and even looking at similar pix and technique on Canon's lighting site), I don't care for the "spotlight" or "gotcha" effect a lot of these photos have. They just look fake and staged. Is there a way to really soften the light and widen the spread of it so it looks more natural while still being able to shoot from a reasonable distance? Would love to hear some comments/suggestions on this...

On your last point - "you will get too little light to focus", I have solved this issue.

I stick a Canon 580 ex II on my camera whose only purpose is to provide focus light. I set it to Master Ettl (remote) and disable the flash. I disable the flash on the 580 ex II - not on the camera body.I then plug in my remote trigger in the PC port of the camera.

With this setting I can focus in complete darkness yet keep all the benefits of off camera flash.

I tried disabling the flash IN CAMERA - does not work. If you use the custom function of canon cameras to do so it will aslo disable the PC flash - not what you want.

I would really like to know how to get those colors in de picture. I understand how to make the background lighter and darker, but I would like to know how to get these nice colors of the sky and water.

Firstly thanks for the amzing amount of information you have put up on this site over the years. It's made a huge difference to my photography. So much so that a recent 'strobist' image of mine was shown on the BBC Evening News here in the UK as part of a feature on what people are doing during the bad weather.

more info in my bloghttp://www.deanrphotography.com/blog/2010/01/igloo-in-bracknell/

Link to the photo now on the BBCs winter gallery.http://www.bbc.co.uk/southtoday/content/image_galleries/weather_jan10_gallery.shtml?20

Thanks again for all the great information. I wouldnt have even had the right equipment let alone the idea had it not been for this site.

Don't know if this link is open, but, I have some questions that I can't wrap my head around.

senario #1: if I'm shooting someone outdoors with off camera strobe. Ambient light metered at face of say, f5.6 at 1/60. I want to add a key light camera left one stop higher, f-8. Now, in a studio, I would have 2 lights and I would meter the combined flash from both to get my exposure of say, f-9.5(not exact but for sake of argument)Question #1: Is it possible to mesure combined flash and ambient with Sekonic L-358 or would I expose for flash exposure?

Question #2: Will changing ISO affect exposure of flash? Ambient?

Question #3: What if I want to darken background ambient.. Using your method, that would change the ambient exposure on subject, i.e.,changing shutter speed from 1/60 to 1/125 will affect overall ambient by 1 stop darker, but not affect strobe. So is it possible to keep same ratio on subject or is second light needed?

Can you explain this paragraph to me please? I've followed everything up to this point and really want to understand this subject inside out. "When the background drops to 1/30th instead of opening up the shutter to compensate, turn the power on your flash down by one stop. (If you were shooting at 1/4 power, move to 1/8.) Now, instead of opening up from 1/60th at f/5/6 to 1/30th at f/5.6, you're opening to 1/60 at f/4 and adjusting your flash to compensate."Am I right in thinking that by turning the flash down one stop you're now exposing your model at f5.6 and not f8?